In a possible terrorist attack Wednesday afternoon, a vehicle rammed into a crowd of pedestrians outside the British Parliament in London. A man then exited the vehicle and stabbed a police officer to death; other officers then shot and killed the assailant.

"Four people have died," said Britain's top counter-terrorism officer, Mark Rowley. "That includes the police officer who was protecting Parliament and one man we believe to be the attacker, who was shot by a police firearms officer."

The attack, which police are treating as terrorism, comes exactly one year after the series of suicide bombings in Brussels that killed 32, along with three bombers.